BUOYANCY-DRIVEN, RAPID EXHUMATION OF ULTRAHIGH-PRESSURE METAMORPHOSEDCONTINENTAL-CRUST

Preservation of ultrahigh-pressure (UHP) minerals formed at depths of 90-125 km require unusual conditions. Our subduction model involves un derflow of a salient (250 +/- 150 km wide, 90-125 km long) of continen tal crust embedded in cold, largely oceanic crust-capped lithosphere; loss of leading portions of the high-density oceanic lithosphere by sl ab break-off, as increasing volumes of microcontinental material enter the subduction zone; buoyancy-driven return toward midcrustal levels of a thin (2-15 km thick), low-density slice; finally, uplift, backfol ding, normal faulting, and exposure of the UHP terrane. Sustained over approximate to 20 million years, rapid (approximate to 5 mm/year) exh umation of the thin-aspect ratio UHP sialic sheet caught between coole r hanging-wall plate and refrigerating, downgoing lithosphere allows w ithdrawal of heat along both its upper and lower surfaces. The intracr atonal position of most UHP complexes reflects consumption of an inter vening ocean basin and introduction of a sialic promontory into the su bduction zone. UHP metamorphic terranes consist chiefly of transformed , yet relatively low-density continental crust compared with displaced mantle material-otherwise such complexes could not return to shallow depths. Relatively rare metabasaltic, metagabbroic, and metacherty lit hologies retain traces of phases characteristic of UHP conditions beca use they are massive, virtually impervious to fluids, and nearly anhyd rous. In contrast, H2O-rich quartzofeldspathic, gneissose/schistose, m ore permeable metasedimentary and metagranitic units have backreacted thoroughly, so coesite and other UHP silicates are exceedingly rare. B ecause of the initial presence of biogenic carbon, and its especially sluggish transformation rate, UHP paragneisses contain the most abunda ntly preserved crustal diamonds.